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Appendix II.                              817

a finishing tool, when cutting screws in the lathe. It is, how-
ever, sometimes used both to start and finish the thread, being
held by the hand throughout the operations. It is easily seen
that such a method has precisely the same objections as stock and
dies, but even to a worse extent, for c drunken; threads, of varying
pitch, are often produced by inexperienced workmen. This is
because the tool has very short directing surface, even though that
surface may have the correct angle.

(5.) By Screwing Machine.—This method is the same in prin-
ciple as stock and dies, and is merely a quicker and more
automatic way of doing the same work. In Fig. 786 the general
form of the machine is that of a short lathe with fixed headstock A,
a clutch B for putting the mandrel in and out of gear quickly, and
a powerful driving gear. The work is held in a concentric chuck
c, and the screwing dies are in themselves a sort of concentric
chuck, operated by the lever E. The mandrel being revolved, the
cutting head D is brought over the work by the pinion and hand-
wheel F until the dies are in position to start: the lever E is then
pulled leftward by the advancing cutters, being partly helped by
the workman's hand at E. -The operation may be repeated until
the stud is of correct diameter, as indicated by the angle of the
lever E. With plenty of lubrication the thread may be cut during
one traverse, and when the dies are blunted they can be recut upon
a master tap placed in chuck c.

v Cutting Long-pitched Screws. — When the thread
angle becomes 45° or over, it may be questionable policy to cut it
in a lathe. Remembering that a screw is formed by an axial
traverse and a rotation, either motion may be called the cut, the
other being the feed; but the latter should always be preferably
the slower or smaller motion. Thus, in the machine for rifling
guns, the traverse being large and the rotation small, the tool is
propelled axially, the holder being a long bar with the tool
secreted in the end, in the manner known as the 'tiger's claw/
from the fact that it is sheathed on entering, and automatically
shot out on the outward or cutting stroke. During withdrawal the
tool bar is rotated by a rack and pinion, the amount of rotation
being fixed by the inclination of a bar along which the rack arm
travels. In such manner any long-pitched screw may be cut.